1. Field of the Invention
The present invention relates to the manufacturing method of a color cathode ray tube, and more particularly to a flat-face type color cathode ray tube which has an outer surface of a panel portion thereof formed into an approximately flat shape.
2. Description of the Related Art
A color cathode ray tube, for example, a color cathode ray tube which is used in a color television set, a color display monitor for an OA equipment terminal includes a vacuum envelope. The vacuum envelope is constituted of an approximately rectangular panel portion which has a phosphor screen including a black matrix (BM) film or a large number of dot-like or stripe-like phosphor pixels on an inner surface thereof, an approximately cylindrical-shape neck portion which houses an electron gun therein, and an approximately funnel portion which connects the neck portion and the above-mentioned panel portion on an axis which is substantially coaxial with a tube axis and includes a deflection yoke on an outer periphery of a transitional region between the neck portion and the panel portion. Further, in the inside of the vacuum envelope, a shadow mask which constitutes a color selection electrode and includes a large number of electron beam apertures is arranged in the vicinity of the phosphor screen in an opposed manner.
This shadow mask uses an aluminum killed steel as a main constituting material thereof. Further, with respect to the shadow mask, along with a recent demand for high definition of the color cathode ray tube, a shadow mask having a small plate thickness has been used. In a color cathode ray tube which adopts the small-plate-thickness shadow mask, a phenomenon in which a portion of the shadow mask is deformed by heat so that an electron beam spot is displaced from a given position on a phosphor screen during a displaying operation, that is, a so-called mask doming phenomenon is liable to easily occur.
As a means to cope with such a phenomenon, along with the improvement of a shadow mask suspension mechanism, a nickel steel alloy of FeNi36 is also used as the constitutional material in view of the thermal expansion coefficient and the physical hardness.
Such a shadow mask is formed as follows. A form in which a large number of electron beam apertures are formed at given positions by etching is blanked in a given shape. Thereafter, the blanked form is formed into a shape using a press such that the shadow mask is constituted of an approximately spherical main surface and a skirt portion which is contiguously formed with a periphery of the main surface and is bent by approximately 90 degrees with respect to the main surface and is used.
Further, recently, along with the popularization of a color television set or a color display monitor having a flat screen type, there is observed a tendency that an outer surface of a faceplate (panel glass) is leveled or flattened with respect to the color cathode ray tube which is used in the color television set and the color display monitor.
FIG. 8 is a schematic cross-sectional view for explaining a constitutional example of a shadow-mask-type color cathode ray tube of a flat panel type. In FIG. 8, a vacuum envelope is constituted of a panel portion 51 which forms a phosphor screen 50 having a black matrix film which consists of phosphor pixels and a non-light-emitting light absorbing material layer on an inner surface thereof, a neck portion 52 which houses an electron gun 61, and a funnel portion 53 which connects the panel portion 51 and the neck portion 52.
The panel portion 51 includes an approximately flat outer surface and a concavely curved inner surface. The phosphor screen 50 which is arranged on the inner surface of the panel portion 51 includes, in general, phosphor pixels which are formed by applying phosphors of three colors of red (R), green (G), blue (B) respectively in a dotted pattern or in a stripe pattern, a black matrix film which surrounds the phosphor pixels and is made of a non-light-emitting light absorption material layer such as carbon, and a metal reflection film which constitutes a metal back layer. Further, a shadow mask 54 is arranged close to the phosphor screen 50. The shadow mask 54 is formed of a nickel steel alloy of FeNi36 by taking a thermal expansion coefficient and a physical hardness into consideration.
The shadow mask 54 is of a self-standing shape-holding type which is formed by a press, wherein a periphery of the shadow mask 54 is welded to a mask frame 57, and the shadow mask 54 is suspended and supported on stud pins 60 which are mounted upright on an inner wall of a skirt portion of the panel portion 51 by way of suspension springs 59. Here, a magnetic shield 58 is fixed to an electron-gun-61-side of the mask frame 57. A deflection yoke 55 is exteriorly mounted on a transitional region between the neck portion 52 and the funnel portion 53 of the vacuum envelope, wherein by deflecting three modified electron beams B which are irradiated from the electron gun 61 in the horizontal direction (X direction) and the vertical direction (Y direction), the electron beams B are scanned two-dimensionally on the phosphor screen 50 thus reproducing an image.
Further, an inner conductive film 62 which is formed on an inner surface of the funnel portion 53 applies a high voltage introduced from an anode button to electrodes which form a main lens of the electron gun 61 and a metal reflection film of the phosphor screen 50. Numeral 63 indicates a reinforcing band, numeral 64 indicates a mouthpiece, and numeral 65 indicates a whole color cathode ray tube.
In the color cathode ray tube having such a constitution, as described previously, the panel portion 51 has the approximately flat outer surface and the concavely curved inner surface. To the contrary, the shadow mask 54 is shaped into the given curved surface by molding the shadow mask form by a press and is curved in conformity with the inner surface of the panel portion 51.
The reason that the inner surface of the panel portion 51 and the shadow mask 54 are curved irrespective of the approximately flat external surface of the panel portion 51 is that the manufacturing method of the shadow mask 54 by a press forming technique can be performed easily and at a low cost.
The curved shape of the shadow mask 54 is a aspherical shape in which radii of curvature are gradually decreased from the center of a main surface to a periphery of the shadow mask 54 respectively along a long axis, a short axis and a diagonal line of the shadow mask 54. The curvatures of the shadow mask 54 of the aspherical shape are determined as follows, for example, wherein an equivalent radius of curvature is set as Re.Re=(z2+e2)/2z 
Here, e: a distance (mm) in the direction orthogonal to a tube axis from the center to an arbitrary peripheral position on a main surface of the shadow mask
z: a falling quantity (mm) in the tube axis direction from the center of the main surface of the shadow mask at the above-mentioned arbitrary peripheral position
Such specification establishes the compatibility between a flat feeling of the screen and the maintenance of a mechanical strength of the shaped shadow mask as the color cathode ray tube
FIG. 9 is a schematic cross-sectional view showing a portion of an essential part of the color cathode ray tube shown in FIG. 8 in an enlarged manner. In FIG. 9, the phosphor screen 50 formed on the inner surface of the panel portion 51 includes three-color phosphor pixels 501 which are formed by applying phosphors of three colors in a dotted pattern or a stripe pattern, a black matrix film 502 which surrounds the phosphor pixels 501, and a metal reflection film 503, wherein the shadow mask 54 is arranged close to the phosphor screen 50 in a state that the shadow mask 54 faces the phosphor screen 50 in an opposed manner.
The three-color phosphor pixels 501 are constituted of a red (R) phosphor pixel 501R, a green (G) phosphor pixel 501G and a blue (B) phosphor pixel 501B. The phosphor pixels 501 are formed on opening portions (window portions) formed in the black matrix film 502 through an exposure step after applying a phosphor slurry on an inner surface of the panel portion on which the black matrix film 502 is formed. The exposure step is performed for every color. Since positions of three light sources 66G, 66B, 66R are different from each other, it is possible to accurately form three kinds of phosphor pixels on the opening portions (window portions) formed in the black matrix film 502 respectively.
In forming the black matrix film 502, a photoresist in a slurry form is applied to the inner surface of the panel portion and, thereafter, the photoresist is exposed, and the photoresist is removed except for photosensitive portions. Then, graphite is applied to the inner surface of the panel portion and the photosensitive portions of the photoresist are removed thus forming opening portions for forming phosphor layers in the black matrix film. In the exposure step for forming the black matrix film, the exposure is performed three times by changing the positions of the light source for forming the opening portions for green phosphors, the opening portions for blue phosphors and the opening portions for red phosphors.
An example of a conventional exposure device is shown in FIG. 10. The exposure device shown in FIG. 10 is an exposure device disclosed in FIG. 2 of patent document 1, that is, patent publication number JP-A-11-167864, wherein numerals used in the drawing are used as it is. In FIG. 10, numeral 1 indicates a panel, numeral 3 indicates a shadow mask, numeral 5 indicates a device body, numeral 6 indicates a light source, numeral 7 indicates a slide mechanism, numeral 8 indicates a first correction lens, numeral 9 indicates a mounting position of two auxiliary correction lenses, numeral 9a indicates a first auxiliary correction lens, numeral 9b indicates a second auxiliary correction lens, numeral 10 indicates an auxiliary correction lens storing chamber, numeral 10a indicates an accommodating shelf, numeral 10b indicates an elevating mechanism, numeral 10c indicates a pullout opening, numeral 11 indicates an auxiliary correction lens rotational drive mechanism, and numeral 12 indicates a second correction lens. According to the explanation of patent document 1, there is disclosed a manufacturing device in which the second correction lens 12 is constituted of a curved-surface type glass having a correction component parallel to the X direction and corrects only components of the positional displacement of the respective phosphor stripes of three colors which are generated during the manufacturing steps and hence, the manufacturing device can correct the positional displacement components which are generated during the manufacturing steps at a low cost coupled with a correction effects of the above-mentioned first correction lens 8.
Further, FIG. 11 shows another example of the conventional exposure device. The exposure device shown in FIG. 11 is an exposure device which is disclosed in FIG. 1 of patent document 2, that is, patent publication number JP-A-2000-268719, wherein numerals are used as it is. In FIG. 11, numeral 1 indicates a light source, numeral 2 indicates a lens system including correction lenses, numeral 3 indicates a main base, numeral 4 indicates a support member, numeral 105 indicates a panel, numeral 106 indicates a panel mounting base, numeral 107 indicates a panel transport base, and numeral 108 indicates a color selection electrode. The lens system 2 includes three sets of lens systems for the R exposure, the G exposure and the B exposure, wherein the lens system 2 is constituted of a flat plate “n” for three colors and correction lenses for respective colors. According to the explanation of this patent document 2, by preparing the respectively independent correction lenses for three colors which are small in use number, it is possible to approximate an optical path from a light source to a panel surface to trajectories of electron beams and hence, it is possible to manufacture a color cathode ray tube with little color slurring compared to the related art.
In the flat panel type color cathode ray tube shown in FIG. 8 in which the panel portion has the approximately flat outer surface, a wall thickness of the panel portion differs between a center portion and a peripheral portion. To reduce a distortion of a screen generated due to the difference in the wall thickness of the panel portion, there has been proposed a novel deflection yoke which has characteristics different from the characteristics of the related art.
Further, in the formation of a phosphor screen of a conventional cathode ray tube having a shape in which the above-mentioned panel wall thickness is substantially equal over a whole panel surface, a system which combines a rotary body lens which reduces the undulation of stripes with a landing correction lens is adopted.